How to slide geometry past an edge?
With the vertices selected, press ShiftV to ensure that you are vertex sliding, not edge sliding. Pressing GG automatically picks between the two operators based on the selection, and you cannot edge slide geometry past adjacent edge loops (you can with vertex slide though). 

Once you are in the vertex slide operator, press C to disable clamp (or hold Alt). 



Note that disabling clamp locks the selected vertices to the edges which they are sliding on when you press C or Alt.



However in this particular situation I think GZ is a faster method, as mentioned by G.Rassovsky.